Base of the 'Taconic or Lou'er C'a)iibri(i.n. — Winchell. 155 
garding the summit. Mureliison at first made the Llandeilo 
the bottom of liis Lower Silurian,* and simply called the non- 
conformable underlying rocks "slaty grauwacke," without 
pushing his descriptions further downward. Later, however, by 
a series of unjustifiable enlargements of his Sihirian system, he 
represented that all the old strata containing a trilobitic or 
brachiopodous fauna should be put in the Silurian. He thus 
made Barrande's "primordial zone" a part of the Silurian and 
theoretically covered all that is known of the faunas of the 
Cambrian. Even Sedgwick, in his final "Tabular view," 
omits the Longmynd slates from his (Cambrian, although they 
had before been included with the remark that "their exact 
place in the general series is doubtful. '"f In another place he 
includes the Longni3"nd rocks, with some doubt, in the Skid- 
daw slate at the bottom of the Cambrian. It was C)nly after 
the visit of Barrande to England in 1851, resulting in the an- 
nouncement of fossils from the primordial zone in that coun- 
try, that careful examinations involving the base of the Cam- 
brian began to be made. A vast amount of labor and of lit- 
erature has been devoted to the British Cambrian since that 
re-examination began. Various life-zones have been estab- 
lished and some deftniteness in the parts has been reached. 
Some of the English geologists, under the lead of Dr. Hicks, 
to whose timely energy and skill is principally due the eluci- 
dation of the Cambrian faunas and stratigraphy in south 
Wales, are satisfied to limit the downward extension of the 
Cambrian at a series of conglomerates and grits which in 
some places seem to coincide with the base of the Olenellus 
zone; while others, who perhaps have with them the majority 
of the working geologists of Great Britain, include in the 
Cambrian those fornuitions wliich Hicks has called I'ebidian, 
Dimetian ami Arvonian, whicli are very largely of eruptive 
characters. If these be embraced in the Cambrian, the bot- 
tom of the Cambrian in the British Isles is an unknown cpian- 
tity. The Olenellus zone, even, has not yet been fully identi- 
fied in Wales where the Cambrian was first studied. It is in 
Pembrokeshire specially that the base of the Cambrian is in 
*Luncl()n and Kdiiibnrsb PliiloS(i]ihiciil maiiaziiic July. 1H:!."i. )>. Ki. Re- 
print ill till' AmKIUCAN (JEOLOfilST. \<)!. V. p. SO. I Silo. 
fl>ritisti I'alc(i/,()ic fossils, p. iv. "M fascimlus. 
